Always use context #0 for now.
Always use context #0 for now.

This hack limits ImageJ to only having one instance at a time, but the
code is closer than before to supporting multiple simultaneous
instances. There are other higher priorities right now, but we can
return to this later.

This used to be revision r3456.

biovoxxel/imagej,TehSAUCE/imagej,TehSAUCE/imagej,TehSAUCE/imagej,biovoxxel/imagej,biovoxxel/imagej